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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        function backDay() {
            let year = prompt('请输入年份')
            let flag = isLeapYear(year);
            if(flag){
                console.log(year + '年份2月有29天');
            }else{
                console.log(year + '年是平年，2月有28天');
            }
            
        }

        function isLeapYear(year) {
            let result = false;
            if ((year % 4 == 0) && (year % 100 != 0) || ((year % 100 == 0) && (year % 400 == 0))) {
                result = true;
            }
            return result;
        }
        backDay();
    </script>
</body>

</html>